In a once-through steam generator the passage of the flow medium usually injected in the form of feed water is forced through the preheater, the evaporator and the superheater by an appropriately powerful feed water pump or feed pump. Thus the heating up of the flow medium to saturated steam temperature, the evaporation and the subsequent superheating occur continuously in one pass, so that no drum is needed. By contrast with a steam generator which is designed for operation with natural circulation, a once-through steam generator can also be operated in the supercritical range at pressures of 230 bar and more. With once-through vessels very high steam pressures can be generated in a relatively small space. Since the quantity of flow medium in the system is relatively small the system has a low inertia and allows a rapid reaction to rapid reaction to load changes.
Fired once-through steam generators with evaporator tubes wound in the form of a spiral around a combustion chamber (so called spiral tubing) are usually designed for a mass flow density of the flow medium carried through the evaporator tubes of appr. 2000 kg/(sm2) at 100% load (full load). In accordance with the previous usual design guidelines the mass flow density in an evaporator with smooth tubes should not fall below a value of around 800 kg/(sm2) at part load, in order to avoid cooling problems on the tube walls through a layering of the flow. This value corresponds, for the above-mentioned full load mass flow densities of 2000 kg/(sm2) to a load value of 40% of the full load. This is then also the load case for which the minimum evaporator mass flow is defined. In startup and low-load operation it is ensured through the feed water regulation that this minimum evaporator mass flow is always fed to the evaporator.
Non-evaporated water, which occurs precisely in startup and low-load operation, is usually separated from steam in a water separator (or separator) connected downstream of the evaporator and carried to a water collection vessel (the so-called collection flask or flask), while the steam is generally fed to a superheater. Often a circulating pump is used to recirculate the separated water and bind it into the feeding water mass flow (or feeding mass flow) before the feed water preheater, also referred to as the economizer, i.e. ultimately carry it back to the evaporator inlet. The evaporator mass flow in this case is made up additively of the feeding mass flow and the circulating mass flow, also referred to as the recirculating mass flow. Such arrangements are known for example from DE 32 43 578 A1, DE 42 36 835 A1 or U.S. Pat. No. 3,412,714.
In a previously usual mode of operation the feeding mass flow is constantly increased on startup, while the circulating mass flow is reduced to the same degree. Consequently, in the above-mentioned example, the circulating pump must be designed for a comparatively high circulating mass flow density of appr. 800 kg/(ms2) corresponding to 40% of the full-load value of the evaporator mass flow density, since in no-load operation or slightly above this level almost the entire evaporator mass flow is formed by the circulating mass flow. This comparatively high design mass flow of the circulating pump leads to the circulating pump having to be dimensioned comparatively powerful and large and accordingly being associated with high procurement costs.